actually it gets a little confusing here. hetrosexual men use the term trap to mean a passable transsexual. so you'd have threads where people would (say for example) post women they'd find attractive, and someone would knowingly or unknowingly post a picture of a transsexual and another person would reply "that's a trap" as a kind of "there is more than meets the eye" commentary.
http://www.urbandictionary.com/define.php?term=It's%20a%20Trap
on this forum however, it seems they've changed the meaning of traps to only refer to crossdressers. not sure why that is, but it's only on this forum.